Vehicle brake systems of the general type under consideration are known from DE 103 36 611 Al, for example. A brake system of this kind has a service brake function and a parking brake function. By means of the service brake function, diaphragm parts of brake cylinders can be supplied with compressed air at a controllable service brake pressure. By means of the parking brake function, spring brake parts of brake cylinders, in particular of combined spring brake/diaphragm brake cylinders including the diaphragm parts, can be supplied with compressed air at an electro-pneumatically controllable parking brake pressure.
Known brake systems also have an overload protection function for the brake cylinders. This will be explained in greater detail with reference to FIG. 1, which shows the prior art.
FIG. 1 shows parts of a brake system 2, which has a first compressed air line 4, in which compressed air at a controllable service brake pressure can be provided. By means of the service brake pressure, a diaphragm part 6 of a brake cylinder 8, which is designed as a combined spring brake/diaphragm brake cylinder, can be supplied with air to apply a brake in order to brake a vehicle having the brake system 2. Discharging air from the diaphragm part 6 releases the brake again.
The brake system 2 also has a parking brake device 10 having a second compressed air line 12. By means of the parking brake device 10, in particular by means of solenoid valves 14 and 16 situated therein, it is possible for a parking brake pressure to be controlled electro-pneumatically and for compressed air at this parking brake pressure to be provided to the second compressed air line 12. For this purpose, the parking brake device 10 has an air supply inlet 18, via which the second compressed air line 12 can be supplied with compressed air in order to increase the parking brake pressure, and also has an air discharge outlet 20, via which air can be discharged from the second compressed air line 12 in order to reduce the parking brake pressure.
In addition to the diaphragm part 6, the brake cylinder 8 has a spring brake part 22. The spring brake part 22 can be supplied with air at the controlled parking brake pressure present in the second compressed air line 12. During this process, compressed air at parking brake pressure passes via a compressed air line 24, an overload protection valve 26 and a compressed air line 28 into the spring brake part 22. The overload protection valve 26 is designed as a shuttle valve or select-high valve with two compressed air inlets and a compressed air outlet. The shuttle valve 26 pneumatically connects the inlet at which the higher of two pneumatic pressures applied to these inlets is present to the outlet.
By means of a spring in the spring brake part 22 of the brake cylinder 8, it is possible to provide a spring force by means of which a brake can be actuated by the brake cylinder 8. In the case where air is discharged from the spring brake part 22, the brake is actuated by means of the parking brake function. Actuation of the brake by means of the spring is particularly suitable for parking the vehicle since the brake thus remains engaged by means of the brake cylinder 8 by virtue of the spring force alone, even if there is a pressure loss in a compressed air line. While the vehicle is being driven, the spring brake part 22 is supplied with air at at least the parking brake pressure. In this case, the parking brake pressure counteracts the spring force, with the result that, above a minimum parking brake pressure determined by the characteristics of the spring, the brake is no longer actuated by means of the parking brake function.
Simultaneous actuation of the brake cylinder by means of the service brake function and by means of the parking brake function could mechanically overload the brake cylinder and/or the brake. The brake cylinder and/or the brake could suffer a fault and fail. Such an overload on the brake through addition of the forces that are provided by means of the spring brake part 22 and by means of the diaphragm part 6 and that actuate the brake cylinder 8 are counteracted by holding an air pressure in the spring brake part 22 continuously at a level at least equal to an air pressure in the diaphragm part 6, i.e., to the service brake pressure. As long as the brake cylinder 8 actuates the brake by means of the spring in the spring brake part 22, an increase in the air pressure in the diaphragm part 6 leads to a reduction in the force provided by the brake cylinder 8 by means of the spring brake part 22. This reduction is achieved through an increase in the air pressure in the spring brake part 22 to the increased service brake pressure.
According to the prior art, an overload protection function of this kind for the brake cylinder 8 and/or for the brake is achieved by means of the overload protection valve 26. More specifically, the overload protection valve 26 selects the higher of the parking brake pressure and the service brake pressure and supplies air at the higher pressure to the spring brake part 22 of the brake cylinder 8 via a compressed air line 28.
The prior art thus provides a valve, namely the overload protection valve 26, which is used solely for the overload protection function. The overload protection valve 26 increases the cost of producing the brake system. Moreover, the overload protection valve 26 requires a certain amount of assembly work, and therefore the overload protection function for the brake and/or brake cylinder 8 of the prior art brake system 2 leads to higher costs compared with a brake system without an overload protection function. In addition, the overload protection valve 26 takes up valuable space in the vehicle, especially on the vehicle frame.